CVE-2026-89897: Linux Kernel CEC Privilege Escalation

CVE-2026-89897 Overview

CVE-2026-89897 is a race condition in the Linux kernel's Consumer Electronics Control (CEC) subsystem. The cec_receive_notify() function reads the exclusive follower pointer without holding the adapter lock. This creates a window where mode changes or follower release can occur concurrently with message delivery, leading to unsafe access of the follower pointer.

The issue affects the media: cec component and has been resolved upstream through multiple stable kernel commits. The vulnerability requires adjacent network access, meaning an attacker must be on the same physical CEC bus (typically an HDMI connection) to trigger the race.

Critical Impact
A concurrent mode change during message delivery can result in use-after-free or inconsistent state within the kernel CEC subsystem, potentially compromising kernel integrity and availability.

Affected Products

Linux kernel — media/cec subsystem
Multiple stable kernel branches receiving backported fixes
Devices exposing CEC interfaces over HDMI (media drivers)

Discovery Timeline

2026-09-16 - CVE-2026-89897 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-89897

Vulnerability Analysis

The Linux CEC framework allows userspace processes to register as an exclusive follower for a CEC adapter. An exclusive follower receives all incoming CEC messages instead of the kernel's default handler. The cec_receive_notify() function checks whether an exclusive follower is registered and delivers messages accordingly.

The defect is that this check and subsequent delivery occur without acquiring the adapter lock. A concurrent thread can change the follower mode or release the follower while cec_receive_notify() is still using the pointer. The fix serializes the no-follower check and the message delivery path against mode changes and release operations by holding the adapter lock.

Root Cause

The root cause is a missing synchronization primitive around access to the exclusive follower pointer. Because CEC message reception runs in a different context than ioctl-driven mode changes, the two paths can execute in parallel. Without lock protection, the reader observed a stale or freed pointer, producing a classic time-of-check to time-of-use (TOCTOU) race.

Attack Vector

Exploitation requires the ability to send CEC messages to the target adapter, typically via an HDMI-connected device on the same physical bus. The attacker must also coordinate timing with a local process that changes follower mode or releases the follower. The attack complexity is high because the race window is narrow and requires specific state transitions on the target.

No public proof-of-concept has been published, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Refer to the upstream commits including Linux Kernel Commit 1924d078 and Linux Kernel Commit 271e57a9 for the exact code changes.

Detection Methods for CVE-2026-89897

Indicators of Compromise

Unexpected kernel oops or panic messages referencing cec_receive_notify, cec-core, or cec-api in dmesg output
KASAN reports flagging use-after-free within the drivers/media/cec/ code paths
Sudden loss of CEC functionality on HDMI-connected media devices following userspace mode changes

Detection Strategies

Enable KASAN and lockdep on test kernels to surface the race and any remaining locking gaps in the CEC path
Correlate kernel crash telemetry across fleets to identify signatures involving cec_receive_notify stack frames
Track kernel package versions against the fix commits listed in the upstream stable trees

Monitoring Recommendations

Ingest journalctl -k and /var/log/kern.log into a centralized logging pipeline for cross-host correlation
Alert on repeated CEC-related kernel warnings originating from the same media device
Monitor for unexpected reboots on embedded Linux systems with HDMI/CEC capable hardware

How to Mitigate CVE-2026-89897

Immediate Actions Required

Inventory Linux systems that expose CEC-capable hardware, particularly set-top boxes, media servers, and embedded devices with HDMI outputs
Apply the stable kernel updates that include the serialization fix for cec_receive_notify()
On devices where kernel updates are delayed, restrict physical access to HDMI ports on production hardware

Patch Information

The fix serializes the no-follower check and message delivery in cec_receive_notify() against follower mode changes and release. It has been backported across multiple stable branches. See the upstream commits: Linux Kernel Commit 4b532b27, Linux Kernel Commit 5c62095a, Linux Kernel Commit ae614f48, Linux Kernel Commit cac57785, Linux Kernel Commit df941e68, and Linux Kernel Commit efc829b4.

Workarounds

Unload the CEC kernel modules (cec, drm_display_helper) on systems that do not require CEC functionality
Blacklist CEC drivers via /etc/modprobe.d/ on servers and workstations without a legitimate media use case
Restrict access to /dev/cec* device nodes to trusted users through udev rules and filesystem permissions

bash

# Configuration example: disable CEC on systems that do not need it
echo 'blacklist cec' | sudo tee /etc/modprobe.d/disable-cec.conf
sudo rmmod cec 2>/dev/null || true

# Restrict access to CEC device nodes
sudo chmod 600 /dev/cec* 2>/dev/null || true